Liquid dispensing mechanisms are used in clinical analyzers to aspirate and dispense various liquids as needed. In the "Ektachem 400".RTM. and/or "Ektachem 700".RTM. analyzers manufactured by Eastman Kodak Company, in addition to aspirating and dispensing sample, a reference liquid has to be aspirated and dispensed when a potentiometric analysis is conducted using ISE test elements. The mechanism that handles the reference liquid is somewhat complex, for several reasons. First, the pump and the removable tip mounted thereon has to move between two operative, "down" positions that correspond to the aspirating position at a reservoir and the dispense position at a dispensing station. Between these positions, it moves to a raised inoperative position. Conveniently, movement between "down" positions is done rotationally wherein the two operative positions are circumferentially positioned on a circle of motion. Second, complexity is introduced because the tip insertion occurs at an angle from the vertical, for clearance purposes. Hence, it is not sufficient to simply raise the pump and its tip, but rather, to raise it along a diagonal.
The mechanism to achieve this on the aforesaid analyzers available from Eastman Kodak Company has been useful and generally satisfactory. However, it has required the use of a large number of complex parts, including a barrel cam, a cam follower mounted to always ride on the cam, the cam follower being connected to a yoke hingedly and rotationally connected to a rack cylinder that drives a pinion gear, a second rack mounted in the pump that is in turn driven by the pinion gear, and a modified Geneva mechanism for controlling the rotation of the barrel cam. Portions of this device can be seen in the Eastman Kodak User Manual entitled "E700/E500 Service Manual", Pub. #XP3080-13, p. 5, 6 & 12, dated 8/86. Because so many moving parts are required, this conventional dispensing mechanism has been more expensive to build, or maintain, than it would be if it used fewer moving parts.
Therefore, prior to this invention, there has been a need for a simpler liquid-aspirating and dispensing mechanism requiring fewer moving parts.